1. Field of the Invention
The present invention relates to the preparation of adipic acid by hydrocarboxylation of pentenic acids, and, more especially, to the preparation of adipic acid by reacting water and carbon monoxide with at least one pentenic acid.
2. Description of the Prior Art
U.S. Pat. No. 3,579,551 describes a process for the preparation of carboxylic acids by reacting ethylenically unsaturated compounds with carbon monoxide and water, in the presence of a catalyst composition essentially consisting of iridium compounds or complexes and an iodine-containing promoter. The ethylenically unsaturated compounds are selectively converted into carboxylic (linear and branched) acids by the subject reaction, preferably conducted in liquid phase, at a temperature ranging from 50.degree. to 300.degree. C. (preferably from 125.degree. to 225.degree. C.) and at partial carbon monoxide pressures advantageously ranging from 5 to 3,000 p.s.i.a. or even from 25 to 1,000 p.s.i.a.
It appears that any source of iridium can be employed, and various sources of iodine-containing promoters are indicated; the I/Ir atomic ratio can vary over wide limits (1:1 to 2,500:1) and preferably from 3:1 to 300:1.
The liquid reaction mixture may contain any solvent which is compatible with the catalyst system, C.sub.2 -C.sub.20 monocarboxylic acids being the preferred solvents.
Example 1 of this '551 patent, carried out using propylene as a starting material, indicates that such a system promotes the formation of branched carboxylic acids (isobutyric).
Example 19 thereof, carried out using 1-hexene as a starting material, confirms the high proportion of branched carboxylic acids thus obtained.
Such a disadvantage (lack of selectivity for linear carboxylic acids) is addressed by U.S. Pat. No. 3,816,489, describing conducting the reaction in question with an I/Ir atomic ratio ranging from 3:1 to 100:1 to obtain predominantly terminal carboxylic acids.
While the fundamental advantage of the above techniques is undisputed in the case of raw materials comprising nonfunctionalized olefinically unsaturated compounds, and in particular in the case of olefins, per se, the adaptation of such techniques to starting materials which, in addition to the site of ethylenic unsaturation, contain a functional group which is reactive under the conditions of the subject reaction, presents many difficulties.
In particular, it has been determined that adaptations conducted using pentenic acids result in at least partial failure, since competing reactions other than that specifically desired occur to the detriment or even complete replacement of the latter, because of the presence of a --COOH functional group on the ethylenically unsaturated starting material compound.
Thus, serious need exists in this art for such a process that simultaneously provides good performance in respect of the carbonylation reaction and an appreciable selectivity for adipic acid.